1. Field of the Invention
The application relates to a method in a wireless communication system and related communication device, and more particularly, to method of handing signaling in a congested core network in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple user equipments (UEs), and communicates with a core network including a mobility management entity (MME), serving gateway, etc for Non Access Stratum (NAS) control. In UMTS system, a Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN.
NAS level congestion control contains the functions: “APN (access point name) based congestion control” and “General NAS level Mobility Management control”. The use of the APN based congestion control is for avoiding and handling of EMM and ESM signaling congestion associated with UEs with a particular APN. With General NAS level Mobility Management control, the MME may also use the reject of NAS level Mobility Management signaling requests under general congestion conditions.
When the NAS level mobility management congestion control is applied due to network overload or congestion, the MME rejects an attach request, a tracking area update (TAU), a routing area update (RAU) or a service request, and the MME replies the UE with a Mobility Management back-off timer.
When congestion control is active, the MME/SGSN stores the Mobility Management back-off timer and the Session Management back-off timer. Then the MME/SGSN enforces the stored back-off time by immediately rejecting any subsequent request from the UE targeting at connecting to the APN before the stored back-off time is expired.
The purpose of a service request procedure is to transfer the EMM mode from EMM_IDLE to EMM_CONNECTED mode and establish the radio and S1 bearers when uplink user data or signaling is to be sent. Another purpose of this procedure is to invoke MO/MT CS fallback or 1xCS fallback procedures. The service request procedure is used when the following conditions occur:                the network has downlink signalling pending;        the UE has uplink signalling pending;        the UE or the network has user data pending and the UE is in EMM-IDLE mode;        the UE in EMM-IDLE or EMM-CONNECTED mode has requested to perform mobile originating/terminating CS fallback or 1xCS fallback;        the network has downlink cdma2000® signalling pending; or        the UE has uplink cdma2000® signalling pending.        
During the service request procedure, the UE will deactivate the EPS bearer locally for which no user plane radio bearer is established. In the congestion case, the network will assign the emergency bearer when the UE initiates the service request procedure for emergency bearer services. However, the network will not be able to assign the user plane radio bearer for non-emergency EPS contexts. As a result, the UE will deactivate all the non-emergency bearer contexts. After the emergency call is finished the UE will try to detach and attach again to get the normal service after the back off timer expires. So the UE will create more signaling.
Further, during the service request procedure, the MME will try to activate the user plane radio bearers and S1 bearers for all active EPS context and send a request to eNB. Then, the eNB tries to allocate the user plane resources and when the eNB fails to allocate radio bearers to EPS context the MME will deactivate the corresponding EPS context locally. In the congestion scenario, when the UE has tries to make emergency call and there is not enough resource available, the eNB may not be able to assign resource to non-emergency EPS bearer. Hence, those EPS context will be deleted in the network and the UE. In the congestion and emergency situation, this increases unnecessary signaling and processing on the MME and eNB. The UE will be considered as attached for emergency bearer services and need to detach and attach again to get normal service, which increase more signaling after the back-off timer expires.